This invention relates to cryogenic apparatus, and, more particularly, to a dewar assembly processed to remove and avoid re-creation of a stable gaseous film boiling layer that otherwise lies between a wall within the dewar and the liquefied gas within the dewar.
A dewar includes an insulated vessel that contains a liquefied gas within its interior. Many electronic devices or other structures require either low temperatures for operation, or have improved performance when cooled. An example of such a device is an infrared detector, which is normally cooled to about liquid nitrogen temperature during service. The dewar maintains that low temperature environment for the device, which is in thermal communication with the liquefied gas in the dewar assembly during its operation.
The dewar assembly may be constructed with the structure to be cooled in place and the liquefied gas later added, or it may be first filled with the liquefied gas and the structure to be cooled added thereafter. In either case, cryogenically cold liquefied gas is contacted to an ambient-temperature wall within the dewar assembly at some point. As the cold liquefied gas contacts the warmer wall, some of the liquefied gas evaporates and forms a boundary layer of gas between the mass of liquefied gas and the wall. If, as is often the case, the wall of the structure remains warmer than the adjacent liquefied gas due to the insulating effect of the reduced thermal flux through the layer of gas, the local boiling continues and becomes a permanent feature of the interfacial region between the liquefied gas and the wall, as long as the liquefied gas is present and there is a heat flux through the cooled structure. The boiling boundary layer becomes a permanent gaseous film boiling layer.
The stable film boiling layer is undesirable in most cases because it acts as an insulator against rapid cooling of the structure. It is more troublesome in dewar systems which contain an electronic device that is to be cooled. The film boiling effect produces thermal and acoustic noise as the bubbles are nucleated, which noise can be detected by the electronic device and results in a decreased signal-to-noise ratio. The noise is particularly of concern during transient operation such as at the startup of the dewar assembly just after the liquefied gas is added.
There is a need for an approach to negate the effect of the permanent film boiling layer. The present invention fulfills this need, and further provides related advantages.